Russia Actually Lights Rockets With an Oversized Wooden Match

This was a question from one of my Twitter followers who'd seen a photo of a culprit in the aborted launch attempt of a Russian Soyuz rocket Saturday, when mission control called the abort just moments before takeoff because of an ignition system problem. Indeed, even some seasoned space geeks were surprised to learn that contraptions fashioned out of birch trees into the shape of a broomstick are placed into the combustion chambers of every Soyuz rocket—including those carrying cosmonauts and astronauts.

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In the West, Russian engineers' talent for finding simple solutions to complex problems is often exaggerated. You may have heard the myth about replacing a million-dollar NASA space pen with… a pencil. Yet the Soyuz really does use something resembling an overgrown match to make sure its engines ignite properly.

R-7 ICBM

Anatoly Zak

The business end of a 310-ton Soyuz rocket sports five main engines with four combustion chambers each, complemented by 12 smaller thrusters to steer the vehicle in flight. It means that at liftoff, a total of 32 nozzles exhaust fire in concert. Ensuring the perfect simultaneous ignition of such a multitude of chambers is no trivial task. Even a single unlit chamber spewing kerosene and liquid oxygen would cause a catastrophic explosion when that fuel hit the waterfall of fire from the nearby nozzles.

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To resolve the problem, 1950s Soviet engineers working on the world's first intercontinental ballistic missile—the R-7, which is an ancestor of the Soyuz rocket—invented a contraption called PZU, a Russian abbreviation for "pyrotechnic ignition device." I found an extremely rare training manual in my archive that offers a rare glimpse into the ingenious design and operation of this contraption.

PZU

Anatoly Zak

After the Soyuz is installed onto the launch pad, technicians working from an access bridge under the pad manually insert wooden sticks in a shape of a T-bar into each combustion chamber. Twenty big sticks go into the main chambers and 12 smaller ones go into the steering engines. At the top of each stick you'll find a pair of pyro-charges, and between pyro-charges there is a spring-loaded brass wire that acts as an ignition sensor. Wires from pyrotechnics and from the sensor run down the stick to an outlet where they are plugged into the launch control network.

When those charges fire inside the combustion chamber, their flame cuts the brass wire and the spring pulls the lose end away, breaking the electric circuit. That's what generates the ignition signal at launch control. Because the firing nozzles of the pyro-charges face each other, the ignition of only one of them ensures that the second one would light up as well (imagine two matches right next to each other). Only after the launch control has confirmed ignition inside of each and every one of the 32 chambers will they open the valves on propellant supply lines, initiating the full combustion. The flimsy wooden rig and a few wires are instantly and harmlessly incinerated in the ensuing inferno.

This setup is not as simple as a regular match, but it is surprisingly reliable and has worked for six decades on hundreds of rockets. Yet on March 12, during the first attempt to launch the new Russian satellite for Earth observation, one of the "matches" failed to fire after the ignition command was issued. It was enough of a problem for the launch control system, which detected the lack of signal from the failed igniter, to call off the propellant injection into combustion chambers. The launch was aborted just a moment before liftoff, and the fully fueled rocket remained safely on the pad.

That's when the wooden PZU demonstrated another amazing quality: a quick turnaround. It took launch personnel in Baikonur Cosmodrome in Kazakhstan less than 24 hours to fix the ignition system, re-fuel the rocket, and go for another attempt on March 13, which was successful. If some more complex ignition system had been involved, the mission would probably have to be postponed longer.

Wooden PZUs can be visible on the nozzles of the Soyuz rocket shortly before liftoff.

Over the years, rocket engineers around the world have invented new schemes to light up rocket engines, with some even involving lasers. However, the long-running Soyuz that still carries people and satellites to orbit relies on its 1950s-era design. By most accounts, PZU is the only surviving component in today's rockets that is made out of wood.

This Friday, if all goes well, Russian cosmonauts Aleksei Ovchinin and Oleg Skripochka and NASA astronaut Jeffrey Williams will to blast off to the International Space Station aboard the Soyuz TMA-20 spacecraft. When they do, it'll be PZU sticks made of Russian birch trees that will light that candle.

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